The present invention relates in general to instruments having an optical window and, in particular, to a new and useful arrangement for purging the window of contaminating substances.
Instruments using optics that are subjected to dirty gas streams have the problem of maintaining cleanliness of the optics so that the readings are not biased by accumulated material that would absorb light. Conventional means of keeping the surfaces clean consist of blowing clean air across a window or lens thereby displacing any dirt ladened gases and maintaining a clean surface. Unfortunately, a source of clean air in the vicinity of the instrument is frequently difficult to find. Filtering the inlet air helps, but also requires filter maintenance. Even good filters on blower inlets allow small particles to be blown on the optical surfaces and will eventually coat, or worse, etch the surface. Compressed air offers no solution as it contains moisture and oil which will be deposited on the surface, and for other reasons such as a cooling effect on the surface, is undesirable to use.
A typical solution to the problem is to introduce purge (clean) air into an annular space 14 between concentric solid wall pipes to tubes 10,12 as shown in FIG. 1 which are supported on a wall 32. Purge air entering the instrument at 20 will not contact the optic surface 22 directly and should form an air curtain at the end 24 of the insolation tube 10 to prevent process dirt from process area 30 from contacting the optics 18 of instrument 16. In practice, this situation does not occur. The air traveling through the annulus 14 aspirates the inside of the isolation tube 10, causing it to be at a pressure lower than the pressure in the sighting pipe 12. This effect causes process gas and entrained dirt to be carried back to the optics 18.
A relevant patent is U.S. Pat. No. 3,310,356 to Borberg, which discloses a device for protecting lenses typically used in a television camera employed in an industrial application. This reference acknowledges that a low pressure area is created in front of the lens due to aspiration. It compensates for this condition by introducing air at a pressure higher than the purge air and allowing this higher pressure air to either go through an aperture in a lens or be transmitted through a shield in front of the lens. In this manner, the effects of aspiration can be reduced since the low pressure air will be directed away from the lens by means of the high pressure air. No isolation tube is used however, and air at two different pressures is needed.
U.S. Pat. No. 2,856,542 to McPheeters shows the use of high pressure air to direct dirt laden air away from the lenses in a transmitter and a receiver of a photo electric system. In effect, the high pressure air forms a wind curtain around the lenses, thus preventing the dirt laden air from impinging thereon. Here again, an isolation tube is not utilized.